1. Field of the Invention
This invention pertains generally to the field of aircraft display units that provide information to the pilot or flight crew of an aircraft.
2. Description of the Related Art
Prior to the advent of electronic displays, analog instruments were developed to provide the pilot with an indication of whether the aircraft was on the desired course. One of these instruments is a course deviation indicator comprised of, in part, “needles” and “dots” for providing the pilot with course information. If the aircraft is on the desired course, the needle would be centered; if not, the needle would deflect in the direction of the desired course, and the numbers of dots over which the needle traversed provided the pilot with a scale from which he or she can judge the amount of the deviation.
For lateral navigation, if the needle deflected to the left, the aircraft is located to the right of the desired course, and the pilot has to correct by steering to the left—towards the needle—to capture the desired course; if the needle deflected to the right, the pilot has to correct by steering to the right and towards the needle. For vertical navigation, if the needle deflected upward, the aircraft is below the desired course (i.e., glide slope), and the pilot has to correct by steering up to capture the glide slope; if the needle deflected downward, the pilot has to correct by steering down to capture the glide slope. If the amount of deviation is one dot from center, the amount of correction is less than if the amount of deviation is two dots from center; likewise, if the amount of deviation is two dots from center, the amount of correction is less than if the amount of deviation is three dots from center.
When flying an aircraft in instrument metrological conditions, that is in a condition when visual references outside of the aircraft are not available (e.g., flying an approach to a runway in low visibility conditions), staying “on the needles” is important. To accomplish this task, however, the pilot must continuously shift his vision between the two needles. Moreover, because the instrument is a separate instrument from the artificial horizon, the pilot must continuously adjust his scan between multiple instruments.
With the development of electronic display units and the ability to establish multiple configurations in one instrument, display units such as electronic primary flight directors can be configured to display both the artificial horizon and course deviation information on the same screen, thereby reducing the scan area of the pilot; however, there is still a requirement of the pilot to process deviation information from two separate scales, where one scale is indicative of lateral deviation and the other of vertical deviation.